Our experiments suggest that, not only does the distribution of tryptamine-immunoreactivity differ from that of serotonin, but the injection of tryptamine intrathecally or i.c.v. in rats causes hyperalgesia, rather than analgesia. Inconsistences in the role of serotonin in analgesia, as determined by pharmacologic studies, may result from simultaneous alterations in the activities of both indolemines. The proposed experiments will test the hypothesis that, in spite of their structural similarity, endogenously formed tryptamine and serotonin play opposite roles in the modulation of pain. Changes in tryptaminergic activity may participate in the development of tolerance to narcotic analgesics or in hyperalgesia during withdrawal from opiates. The long-term objective is to determine whether tryptamine in the CNS plays a role in pain or in any action of narcotic analgesics, and not to prove or disprove a transmitter role of tryptamine. The first specific objective is to characterize the distribution of tryptamine-immunoreactivity using immunohistofluorescence and its relation to the concentration and turnover of tryptamine in brain extracts, as measured using high pressure liquid chromatography (HPLC) with fluorometric detection, focusing on areas of known pain-pathways in the CNS. The second specific objective is to differentially manipulate the metabolisms of tryptamine and serotonin (using precursor loading and enzyme inhibitors) in order to correlate changes produced in the concentration or distribution of tryptamine, relative so that of serotonin, with changes in pain-thresholds. The tail-flick, hot plate and flinch jump assays will be used to reflect changes in pain-thresholds in rats. The third specific objective is to determine whether the concentrations or distributions of tryptamine and serotonin in the CNS are differentially affected by manipulations known to affect the activity along pain-pathways, such as acute vs. chronic treatment with narcotics, naloxone-precipitated withdrawal, acute pain produced by electric shock, and chronic pain produced by a polyarthritic condition. This research will provide a better understanding of pain-mechanisms, a necessary prerequisite in the long term development of pain-therapy as well as in the control of tolerance to and withdrawal from narcotics.